The present invention relates to an ignition timing control device in an internal combustion engine for automobiles.
In an internal combustion engine the combustion speed of fuel-air mixture in the cylinder chamber of the engine lowers as the throttle valve is closed so as to reduce the amount of fuel-air mixture introduced into the cylinder chamber. In view of this an internal combustion engine generally employs a vacuum advancer which advances ignition timing in accordance with the intensity of the vacuum in the intake passage of the engine.
The vacuum advancer generally comprises a diaphragm means and a mechanism adapted to be driven by said diaphragm means so as to advance ignition timing when vacuum is supplied to the diaphragm chamber of the diaphragm means. The diaphragm chamber is generally connected with a vacuum port provided in the intake passage of the engine, said intake passage incorporating a throttle valve and supplying fuel-air mixture to the cylinder chambers, the vacuum port being positioned at the upstream side of the throttle valve when it is substantially closed so as to effect idling operation of the engine and being positioned at the downstream side of the throttle valve when it is opened beyond a predetermined opening. Such a vacuum port is generally called an advancer port. In this conventional vacuum advancer, therefore, no vacuum advancing of ignition timing is effected in idling operation, and only when the throttle valve is opened beyond a predetermined opening so as to operate the engine in an at least partially loaded condition is the vacuum advancer put into operation so as to advance ignition timing in accordance with the intensity of the vacuum generated in the intake passage. A substantial advance of ignition timing by the vacuum advancer is effected so far as the throttle valve is opened up to approximately three fourths of full opening, and when the throttle valve is opened beyond this limit, the vacuum advancing of ignition timing is substantially reduced to zero because in this case the vacuum port is substantially exposed to atmospheric pressure.
A conventional vacuum advancer of the abovementioned structure provides a satisfactory vacuum advancing performance of ignition timing in conventional internal combustion engines which incorporate no exhaust gas purifying system. However, in a modern automobile engine which incorporates an exhaust gas purifying system and operates at a relatively high rotational speed even in idling operation, it is desirable that a certain degree of vacuum advancing of ignition timing should be effected even in idling operation, because by this arrangement it is possible to increase the power output of the engine per unit amount of fuel, and therefore it is possible to improve fuel consumption in idling operation.
Further, it is known that when an automobile is driven at high altitude, power output of the engine lowers due to a reduction in the charging efficiency of the engine caused by lowering of atmospheric pressure, while the knock limit advantageously increases, so that in high altitude operation it is possible to advance ignition timing by an additional amount without causing knocking of the engine. Therefore it is contemplated that by incorporating such an additional advance of ignition timing in high altitude operation it is possible to compensate for the reduction of output power due to lowering of atmospheric pressure.